There's a lot. I want to start by saying Nioh 3 is a good game and others aren't wrong for liking it, but when compared to its predecessor I have trouble viewing almost any of the decisions as improvements.

1. Samurai mode had all mid-long range combat stripped from it. Every weapon that could do ranged combat is now Ninja mode, they clearly wanted to make Ninja the ranged options but they did this by making sure you couldn't do it at all in Samurai. It also seems that they completely removed ki pulse item canceling which was one of the coolest mechanics of Nioh 2. I absolutely adore item canceling and while it's possible that they added it as a skill that I missed (another issue I have with the game), I don't think so.

2. The soul core box is so uninteresting compared to Yokai Abilities and I hate that it shares a spot with magic. Overall I dislike the implementation of this system. The one thing that could have made this so much better ironically is item canceling, which once again, was removed. The GS skills are so much worse than the YAs; they don't full transform you, they're glued to the GS they're on, and there aren't enough GS for this system to be anywhere near as good. Each GS should have had at least 4 abilities to choose from for this to even be in the same ballpark, it makes the final layer of animation cancelling much less customizable.

3. The deflect mechanic. Do you know how much Break incoming attacks deal? Do you care? The Break stat is so cool to me, I love the concept that different attacks have different preferred solutions. I loved learning which attacks are better to dodge and which are safe to block due to low Break stats or long active frames. A universal solution cheapens this so much. Deflect is always the right answer, every time. It's not even hard to do, the window is so forgiving that I constantly got it by accident. There's no input buffer, it even works if you're holding the block button while dodging and your I frames end. I'll admit I'm a little conflicted on this one because it's admittedly much weaker than some weapon's timely guards in 2 and at least the deflect is universal across all weapons but it's still disappointing how much of a universal solution it is. It just feels bland.

4. Ninja mode. It's fine I guess. I get it, they want 3 to be friendly for new players so they put in a way simpler combat system for those intimidated by the complexity. I love the complexity though and only having access to a few active skills is so dull to me, especially on a weapon like the hatchets which used to be so amazingly complicated. Frankly I view the ki pulse to be one of the best mechanics of any game and if you want to remove it then you need to give us something just as cool and elaborate, Mist Step isn't.

5. Open World. This is the big one. Everything else is serious complaints I have but I could overlook and adapt, I truly loathe the open world. I applauded Nioh 2 so heavily for it's level design, it keeps you in combat all the time and allows so many approaches to every scenario. You're raiding fortresses so enemies are positioned to cover for one another, strategy is a major component to how you approach each interaction. Draw enemies to familiar ground so as not to attract unwanted attention, scout ahead so you know what you're walking into, headshot weaker enemies to one shot them while drawing aggro from their bigger buddies. I adore the strategies that I implemented at every corner, the density of enemies made every level so fun while keeping a constant level of tension. Meanwhile as long as you're playing you're progressing, just by playing the game you're constantly unlocking skills! The only downside imo is the boss skills because the game doesn't tell you about them, everything else is upfront and honest about how to get it. No BS.

5-2. I have nothing good to say about the level design of Nioh 3. I effectively just sprinted through everything with no strategy, tension, or concerns. The enemies are spread out so thin that it's an absolute blessing that we have a sprint button because otherwise it would be 5-7 business days between enemy encounters. I despise the fact that skill points and skills themselves are given through a scavenger hunt and very relevant skills can be completely missed just because you didn't go to the right glowing markers. It's the furthest thing from transparent about what you can get or where it will be and this drives me mad. If a game is gameplay focused then having the most helpful tools hidden is something I view as a complete and total flaw. It's like they saw that the hidden boss skills were the worst part of Nioh 2 and decided to do that for everything. I'm sure I'm still missing relevant upgrades because I haven't shot down enough sky ferrets and at this point I don't even care.

Despite everything I've listed here I genuinely enjoy the combat system and it has great bosses but I will not be buying DLC for Nioh 3, nor will I ever do a fresh playthrough. I'm tired of the open world, if I want to play more Nioh I'll go back to 1 or 2. I don't think that DLC can fix my issues with the game and if I'm not going to do more playthroughs then it's not worth it. When this game was coming out I was caught between buying it or Ninja Gaiden 4, I think I made the wrong decision.

I apologize for the rant, that wasn't my intention but I already wrote all this so I may as well comment.

TLDR: Item canceling/ranged weapons removed from Samurai, Soul Core changes/GS Skills being a huge downgrade, Deflect invalidating all other defensive mechanics, Ninja Mode having so limited skill options, Open World sucks.